What is Roof Sheathing? (Definitions + Applications) for 2023

Roof Sheathing (Blog Cover)

Roof sheathing is a layer of boards fastened to the joists and trusses. The purpose of the sheathing is to provide a solid, continuous surface onto which your shingles and other roofing components can be securely attached. Roof sheathing, also known as roof decking, lies just under the top layer of shingles.

The roof sheathing will come in square panels of wood that will be made from either oriented strand board or plywood. Take a look at a breakdown of both types of wood:

Types of Wood for Roof Sheathing

Oriented Strand Board

Oriented Strand Board (or OSB) is made by compressing thin wood strands together and binding them with adhesive. Then, the strands are hotly compressed to form a strong bond. 

Each strand layer is aligned perpendicularly to the subsequent layer for maximum strength. OSB is a popular choice because it is inexpensive and provides adequate power for roof decking. 


Plywood is a more expensive option (generally) than OSB and a more robust material. Plywood is made by gluing together multiple layers or plies of wood and compressing them tightly. 

Plywood tends to be the more potent option for roof sheathing, which is higher. However, plywood tends to be heavier than Oriented Strand Board because of its more robust composition. Plywood sheathing is generally best when the shingle material is heavy, like slate or concrete roofing shingles and tiles. 

Still, whether Oriented Strand Board or plywood is used for the roof sheathing, its purpose remains the same: to provide a solid, continuous surface onto which shingles and upper roofing components can be attached. 

The Purposes of Roof Sheathing

Roof sheathing isn’t installed exclusively to attach shingles. It also serves a few critical periphery roles for the roofing system and structure. Take a look at essential functions that roof sheathing serves:

Flame Retardation

No roof sheathing material will be 100% fireproof, but your sheathing can slow the spread of flames in case of a fire. In addition, roof sheathing panels can be coated with notable flame retardant films to offer excellent protection against the spread of fire. 

Having this specially treated sheathing may give firefighters the extra time they need to save the lower structure of the building in case of a fire.

Leak Protection

You can also think of roof sheathing as a crucial yet straightforward additional defense against leaks. For example, it effectively creates an extra layer between the shingles and the roof’s frame. 

Many sheathing materials can be weatherproofed as well to provide extra protection. In this sense, it can also protect your home against severe water damage. 

Weight Distribution

Roof sheathing helps evenly distribute weight when it is placed on the roof. In addition, the panels create a surface that covers the strongest point of your roof – its trusses. 

Weight distribution alleviates the stress that the trusses would otherwise bear all on their own when debris, snow, water, or even foot traffic occurs on your roof. Your shingles will also add considerable weight to your roof. 

Roof sheathing helps spread that weight out across the entire roofing plane to ensure no one spot is overloaded. 

When Do You Need New Roof Sheathing?

Roof sheathing provides strength to the structure, but it is not invulnerable. There are a handful of instances when you must replace it, including:

Water Damage

If you see signs of water damage on your ceiling, like discoloration, sagging, or actual leaks, it’s a probable sign that the roof sheathing has been compromised. 

Exterior Sagging

You may notice visible sagging on the outside of your roof if your sheathing is damaged and needs to be replaced. 


You may be able to see your roof sheathing from your attic. If so, it’s crucial to inspect it every once in a while for signs of rot. In addition, it’s vital to replace rotted roof sheathing as soon as possible to prevent a significant roofing catastrophe.

Roof Punctures

It probably doesn’t need to be said, but roof punctures are causes for sheathing replacement in the interest of being thorough. Punctures may be subtle sometimes, though. 

For example, try going up to your attic in the dark (carefully) during the day and seeing if you can detect beams of light coming through small punctures in your roof.